1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a semiconductor device developing a negative resistance and having a superlattice structure.
2. Description of the Prior Art
Recently, there have been proposed high frequency devices having a superlattice structure according to various structures and principles. Among these devices is a device developing a negative resistance, namely, a negative resistance device capable of effecting an oscillation in a high frequency which exceeds the frequencies of the conventional devices. There has been recently proposed a negative resistance device using a real space transition in a superlattice structure formed by use of a heterojunction of GaAs/Al.sub.x Ga.sub.(1-x) As configuration (Applied Physics Letters, Vol. 44, No. 11, p. 1054, 1984). FIG. 11 shows a cross-sectional configuration of an example of such a device. Namely, the configuration includes an undoped GaAs buffer layer 12, an n-type Al.sub.0.36 Ga.sub.0.64 As layer 13, a superlattice layer 14, an n-type Al.sub.0.36 Ga.sub.0.64 As layer 13, an n.sub.+ -type GaAs layer 15a-15b for contact, and two ohmic electrodes 16a-16b formed respectively over a half insulated GaAs substrate 11. As a phenomenon of this device, a negative resistance appears when voltages applied to the electrodes 16a-16b are increased. In this structure, the superlattice layer 14 is the most important portion to develop the negative resistance effect. This layer 14 has a structure in which a plurality of unit structures 140 of the superlattice layer as shown in FIG. 12 alternately lie on each other. The unit structure of the superlattice layer 14 comprises an 80 .ANG. thick doped Al.sub.0.3 Ga.sub.0.7 As barrier layer 141, and 80 .ANG. thick GaAs layer forming a first quantum well layer 142, and an 80 .ANG. thick Al.sub.0.06 Ga.sub.0.94 As layer forming a second quantum well layer 143. Here, the first and second quantum well layers 142 and 143 each have the same impurity concentration, Si-doped 10.sup.15 cm.sup.-3.
FIG. 13 shows an energy band diagram of the conduction band in the unit structure of the superlattice. The operation mechanism of the device will be described below. In this structure, two kinds of quantum well layers 22 and 23 having different properties with respect to the conduction electrons (corresponding to the reference numerals 142 and 143 of FIG. 12, respectively) are alternately formed with a barrier layer 21 (corresponding to the reference numeral 141 of FIG. 12) therebetween. Where the thickness of the barrier layer 21 is about 80 .ANG. which allows electrons to pass therethrough. Here, the quantum well layers 22 and 23 each are sandwiched between two barrier layers 21. When the conduction band in the quantum well layer is quantized, the ground state of electrons in the quantum well layer 22 is set to, for example, energy level 221 and the ground state of electrons in the quantum well layer 23 is set to energy level 231. Since the energy level 221 is lower than the energy level 231, the conduction electrons are substantially at the energy level 221 in the equilibrium state and hence conduction is dominated by the electron mobility in the quantum well layer 22. However, when the energy of an electron accelerated in an electric field generated by an increasing voltage exceeds the energy difference between the energy levels 221 and 231, tunnel effect takes place as indicated by an arrow in FIG. 13, and hence electrons are allowed to pass through the barrier layer 21 and to move from the quantum well layer 22 to the quantum well layer 23.
On the other hand, as can be seen from the relationships between the electron mobility and the AlAs mol fraction in the Al.sub.x Ga(1-x). As of FIG. 14, the electron mobility in the GaAs is more than that in the Al.sub.0.06 Ga.sub.0.94 As. Consequently, when the composition of the quantum well layer 23 is set to Al.sub.0.06 Ga.sub.0.94 As, the mobility of the electrons transferred to and situated in the quantum well layer 23 by virtue of the tunnel effect is apparently lowered. The result is that a negative resistance, as shown in FIG. 15, is developed.
Conventional negative resistance devices are attended with various problems. In the conventional example, when the thickness of the quantum well layer is about 80 .ANG., the energy state of the electrons in the well layer is quantized. This is called a quantum well. When the tunnel effect is desired to be used between quantum levels of the different quantum wells, the content of Al (mol fraction of Al As) in the quantum well and the layer thickness must be precisely controlled as parameters to determine the quantum level of each quantum well. In a crystal growth method (for example, the molecular beam epitaxial growth method, MBE) for obtaining the superlattice structure, the control of the crystal composition (the mol fraction of Al As in this case) is more difficult than the control of the thickness. The thickness can be controlled to the level of a single-atom layer. However, fine control of the crystal composition cannot be easily implemented. Moreover, in the conventional example, crystal growth utilizes a crystal having two different kinds of aluminum compositions. To achieve a precise control of the two kinds of aluminum compositions, two aluminum cells are required in the MBE method. However, the use of two aluminum cells leads to a loss of controllability.
In addition, it is clearly favorable with respect to the performance of the device in the conventional example from a point of view of the operation principle that a greater difference exists between the electron mobility of the quantum well layer 22 and the electron mobility of the quantum well layer 23 of the FIG. 13. For example, in a report of the conventional example, the difference between the electron mobility of GaAs and Al.sub.0.06 Ga.sub.0.94 As is used. However, as can be seen from the conventionally reported data of FIG. 14, a great difference cannot be expected. Moreover, to obtain a great value of the difference between the electron mobilities, the relative amount of aluminum in the quantum well layer must be increased. In addition, as the relative amount of aluminum in the quantum well layer is increased, it is necessary to increase also the relative amount of aluminum in the barrier layer. However, in the general Al.sub.x Ga.sub.(1-x) As, as the amount (x) of aluminum increases, the amount of impurity atoms having deeper energy levels increases and hence the crystallizability of the device is decreased. On the other hand, when the amount (x) of aluminum in the barrier layer becomes greater, the heterojunction boundary of the GaAs quantum well layer not containing aluminum is deteriorated.
Consequently, the electron mobility in the quantum well layer 22 is lowered and hence a decrease of the operation speed of the device and the like may possibly be caused. This decrease leads to problems associated with the manufacturing of the devices and with their performance.
Moreover, in the conventional example, only the lowest levels 221 and 231 are considered for the potential energy of the quantum levels formed in the quantum well layers 22 and 23, respectively. In general, a plurality of quantum levels having the higher energy exist in the quantum well layer.
For a semiconductor device using a transition process of electrons between the quantum well layers 22 and 23, applied as a high frequency device (up to 100 GHz), such as when the limit of the frequency in the transition process is to be considered, the determination of the structure with consideration to such quantum levels is essential. The consideration of the quantum levels is missing in the conventional example.